麦盖提斜坡奥陶系碳酸盐岩碳氧同位素特征及其意义

Carbon and Oxygen Isotope Characteristics and Its Significance of Ordovician Carbonates in Yubei Area of Maigaiti Slope

依据实测的塔里木盆地麦盖提斜坡玉北地区41个碳酸盐岩碳氧同位素数据,结合岩石学方法,研究了碳氧同位素的组成、演化及其地质意义。数据显示,δ^(13) C值主要分布在-2.6‰~0.7‰,均值为-1.0‰;δ^(18) O值分布在-9.4‰^-3.5‰,均值为-6.9‰。玉北地区古盐度为118.39~126.34,平均为121.94。奥陶系碳酸盐岩淡水改造作用明显。碳氧同位素的组成和演化不但可以指示沉积环境,而且还与生物生产率以及古海平面变化呈正相关性:δ^(13) C的低值对应于局限台地台内滩亚相沉积环境;δ^(13) C的高值对应于开阔台地滩间海、台内滩亚相沉积环境。碳氧同位素组成还对成岩环境有明显响应:鹰山组δ^(13) C与δ^(18) O均向高负值偏移,表明经历过强烈的表生岩溶作用;蓬莱坝组δ^(13) C低—中负值,δ^(18) O表现为高负值,在白云岩储层中可见鞍状白云石及燧石,主要为深埋藏成岩环境;良里塔格组同位素特征为δ^(18) O高负值,δ^(13) C低正值,并且在进入埋藏岩溶阶段之前还经历过风化壳岩溶作用。

Based on 41 analyzed results of carbon and oxygen isotope,we have discussed the compositions,evolution and geological significance ofδ^18O andδ^13）C recorded in the carbonates of Ordovician in Yubei area of Tarim basin,using petrologic method.The result shows thatδ^13 C values range from-2.6‰ to 0.7‰,with average value of-1.0‰;andδ^18O values range from-9.4‰ to-3.5‰,with average value of-6.9‰.The values of paleosalinity range from 118.39 to 126.34,with average value of 121.94.The transformation of freshwater in Early Ordovician is strong.The isotopic composition and evolution not only indicate the sedimentary environment,but also show positive correlation with bioproductivity and palaeo-sea level changes：low value ofδ^13C corresponding to platform shoal of restricted platform environment,high value ofδ^13C corresponds to interbank sea and platform shoal of open marine platform.Different carbon and oxygen isotopes composition also indicates different diagenetic environments.Theδ^13C andδ^18O value of Yingshan Formation are high negative,which suggest that carbonates experienced strong epigenic karstification.The values ofδ^13C are low to medium negative andδ^18O are high negative in Penglaiba Formation,and saddle dolomite and flint occur in the dolomitite reservoir,indicating that carbonate experienced buried diagenetic environment.Theδ^13C values in Lianglitage Formation are low positive andδ^18O are high negative,which suggest that carbonate experienced buried diagenetic environment.